The next version (v24) of DD-WRT should make this possible but right now there is only one real secure way to allow public & private access to your wifi at the same time using a single WRT54G: (That I can think of.)

1. If you have some extra space & an old PC, setup that Netgear as a dedicated public hotspot using something like PublicIP.net

2. Seperate the wireless network from the wired LAN ports on the WRT using the method mentioned above but then run a cable from a LAN port on the WRT to a LAN port on the Netgear. Configure the Netgear with a different SSID, channel & security settings from the WRT and then your trusted clients can connect to the Netgear like it was an access point and have access to the private wired network. (You'll need to use appropriate security such as WPA on the Netgear of course.)